The host defense mechanism against P. carinii is poorly understood. We plan to examine host responses to a purified P. carinii surface antigen, gp116, using an animal model of P. carinii pneumonia. Multiple approaches will be utilized. We will immunize rats with the antigen, and look for the development of antibodies as well as T-cell responses. We will also evaluate this antigen as a vaccine candidate to prevent or decrease the severity of P. carinii pneumonia in immunosuppressed patients. We will further look in vitro and in vivo immunization as well as to natural infection. The goal of these studies is to better understand immune response to P. carinii with the hope that such information can be used to prevent or control diseases.